Between 25% and 50% of women under the age of 25 become infected with the human papillomavirus (HPV) after having unprotected sex. In fact, it is the most common sexually transmitted disease (STD) in the world. Although the population is often more concerned about the risk of contracting AIDS, human papillomavirus infection should not be taken as a joke either. Especially since this DNA virus is capable of inducing tumors. The good news is that HPV vaccines prevent infection and avoid its devastating consequences.
HPV has more than one face. Specifically, about 200 viral types have been identified, which vary in the sequence of the L1 surface protein. Each of them is assigned a number (for example, HPV16). Some viral types infect skin cells, while others prime on mucous membranes.
It is better prevent
After infection, antibodies are generated that bind to the viral type responsible for the infection. The downside is that they do not remain in the body for long (as, unfortunately, also happens in COVID-19), since the plasma cells that produce them progressively disappear, and with them the antiviral antibodies. This implies that neither the antibodies generated by natural infection nor those generated by vaccines are capable of eliminating established infections. However, they can prevent them (so-called prophylactic vaccines).
Does this mean that it is impossible to eliminate an established infection? Absolutely. In most cases the virus is shed spontaneously. Although the mechanisms responsible for this disappearance are not yet fully understood, antiviral cytotoxic T lymphocytes play an essential role in destroying infected cells.
Unfortunately, a long-lasting infection sometimes occurs. When this occurs, and the viruses are of high oncogenic risk, they can produce premalignant lesions that potentially evolve into invasive tumors in the cervix (cervix) or other mucous membranes (vagina, vulva, anus and penis).
Shots at puberty
The goal of HPV vaccines is to generate long-half-life plasma cells that produce antibodies for tens of years. For this reason, it is preferred to vaccinate before sexual intercourse – the route by which the virus is usually spread – is preferred, although invasive cancers do not develop until years after an unresolved infection.
And are only women vaccinated? In Spain, yes. As various Spanish scientific societies explained in 2008 in an agreement regarding HPV vaccination, the long-term end goal of HPV vaccines is the prevention of invasive cervical cancer (CCU). This tumor is diagnosed in about 2000 women in Spain and more than half a million in the world each year.
However, recently some countries inside and outside the European Union have extended vaccination to males between 9 and 14 years old. This measure is aimed at also eliminating other cancers related to human papillomavirus infection, some of which affect males. In addition, it favors herd immunity, reducing the probability of the virus spreading.
To create the HPV vaccines, a technique is used to obtain aggregates of the L1 protein, free of viral DNA and other viral components and therefore non-infective. When administered to the skin to girls 9 to 12 years of age (two doses), neutralizing anti-L1 antibodies are generated that reach tissues exposed to the virus and remain in blood and tissues for years.
As with other vaccines, HPV vaccines can achieve levels of these antiviral antibodies higher than those seen in natural infection. Antibodies generated against a certain genotype may not bind to other human papillomaviruses whose sequence in L1 is different (no cross-reaction between different genotypes). For this reason, all currently marketed vaccines contain the most frequent high-risk genotypes in the population (genotypes 16 and 18), which are responsible for approximately 70% of cervical cancers worldwide.
Some of them also incorporate low-risk genotypes responsible for genital or high-risk warts that have been identified in a lower proportion of tumors.
How effective is the vaccine?
The long period of time between infection and the appearance of tumors meant that, at the time of approval for the use of these vaccines, only data were available showing that the vaccines prevented the development of long-term infections and low-grade premalignant lesions. or high grade (surrogated indicators).
But only a few weeks ago new information arrived. The first article was published showing that the risk of cervical cancer in girls vaccinated before the age of 17 in Sweden was reduced by 88%. Although they are very important data, it is necessary to continue analyzing whether this protection lasts more than the 10 years analyzed. And also if it continues to protect 20-40 years after being vaccinated, the moment when the incidence of these tumors is higher.
Are safe
As for its safety, it is not in doubt. Vaccines have a characteristic that makes them different from other therapeutic measures, and that is that they are administered to healthy people (in childhood or in adulthood). The appearance of symptoms in the days after vaccination makes it intuitive to associate them with the use of the vaccine. For this reason, the safety of vaccines is monitored more rigorously than that of many other drugs.
The analyzes of national and international organizations have repeatedly shown that the safety of the HPV vaccine is similar to that of others in use already established when measures adapted to the age of vaccination are adopted (remaining seated or lying down for a short period of time after vaccination).
Everything indicates that the analysis of the new data that appears in the future will allow us to know the real efficacy of vaccines in the prevention of cervical tumors. As well as the cost / benefit of its extension to male adolescents throughout the world. What seems indisputable is that a decrease in their price and transparency in setting the cost of drugs would facilitate their use and expand their impact on the health of the population.
Pedro Aparicio Alonso, Professor of Immunology. University of Murcia, University of Murcia
This article was originally published on The Conversation. Read the original.
